Table 34.5 ). The driving force for water reabsorption is a transtubular osmotic gradient established by reabsorption of solute (e.g., NaCl, Na+-glucose). Reabsorption of Na+ along with organic solutes, HCO3 − , and Cl− from tubular fluid into the lateral intercellular spaces reduces the osmolality of the tubular fluid and increases the osmolality of the lateral intercellular space. The osmotic gradient across the proximal tubule established by these transport processes is only a few mOsm/L (
Fig. 34.4 ). Because the proximal tubule is highly permeable to water, primarily owing to expression of aquaporin water channels (AQP1) in the apical and basolateral membranes, water is reabsorbed across cells by osmosis. In addition the tight junctions in the proximal tubule are also water permeable, so some water is also reabsorbed across the paracellular pathway between proximal tubular cells. Accumulation of fluid and solutes within the lateral intercellular space increases hydrostatic pressure in this compartment. The increased hydrostatic pressure forces fluid and solutes into the capillaries.
2In addition, protein oncotic pressure in the peritubular capillaries (πpc) is elevated because of the process of glomerular filtration (see ). The elevated πpc facilitates uptake of fluid and solute into the capillary.
•Fig. 34.4 Routes of reabsorption of water and solute across the proximal tubule. Transport of solutes, including Na+ , Cl− , and organic solutes, into the lateral intercellular space increases the osmolality of this compartment, which establishes the driving force for osmotic reabsorption of water across the proximal tubule. This occurs because some Na+,K+-ATPase and some transporters of organic solutes, HCO3− , and Cl− are located on the lateral cell membranes and deposit these solutes between cells. Furthermore, some NaCl also enters the lateral intercellular space via diffusion across the tight junction (i.e., paracellular pathway). An important consequence of osmotic water flow across the transcellular and paracellular pathways in the proximal tubule is that some solutes, especially K+ and Ca++ , are entrained in the reabsorbed fluid and thereby reabsorbed by the process of solvent drag.
reabsorption in the proximal tubule. The reabsorbed fluid is slightly hyperosmotic relative to plasma. However, this difference in osmolality is so small it is commonly said that proximal tubule reabsorption is isosmotic (i.e., ≈67% of both the filtered load of solute and water are reabsorbed). Indeed, there is little difference in the osmolality of tubular fluid at the start and end of the proximal tubule. An important consequence of osmotic water flow across the proximal tubule is that some solutes, especially K+ and Ca++ , are entrained in the reabsorbed fluid and thereby reabsorbed by the process of solvent drag (see
Fig. 34.4 ). Reabsorption of virtually all organic solutes, Cl− and other ions, and water is coupled to Na+ reabsorption. Therefore changes in Na+ reabsorption influence reabsorption of water and other solutes by the proximal tubule. This point will be discussed later, notably in , and is especially relevant during volume depletion when increased Na+ reabsorption by the proximal tubule is accompanied by a parallel increase in HCO3 − reabsorption, which can contribute to metabolic alkalosis (i.e., volume contraction alkalosis).